1. Field of the Invention
The present invention relates to a display device, and more particularly to an OLED display device using thin film transistors formed on a transparent substrate made of glass, plastic, or the like and a driving method thereof. In addition, the present invention relates to an electronic equipment using the display device.
2. Description of the Related Art
In recent years, a mobile telephone is widely available as communication technology develops. In the future, moving picture transmission and a larger amount of information transfer are further expected. With respect to a personal computer, products for mobile applications are manufactured due to a reduction in weight thereof. A large number of information devices which are called personal digital assistants (PDAs) starting with an electronic notebook are also manufactured and becoming widely available. In addition, with the development of display devices and the like, the majority of portable information devices are equipped with a flat display.
Further, according to recent techniques, those information devices tend to use an active matrix display device as a display device used therefor.
According to the active matrix display device, a TFT (thin film transistor) is located in each pixel and a screen is controlled by the TFTs. Such an active matrix display device has advantages in that it achieves higher definition and improved image quality and can handle moving pictures, as compared with a passive matrix display device. Thus, in the future, it is considered that a display device for the portable information device will be changed from the passive matrix type to the active matrix type.
Also, of active matrix display devices, in recent years, a display device using low temperature polysilicon is commercially available. According to a low temperature polysilicon technique, in addition to a pixel TFT composing a pixel, a driver circuit can be simultaneously formed using TFTs in a peripheral region of a pixel portion so that it makes a significant contribution to miniaturization of the device and reduction in consumption power thereof. Accordingly, in recent years, the low temperature polysilicon display device is becoming an essential device for the display portion or the like of a mobile device whose application fields are expanding remarkably.
Also, in recent years, a display device using an organic electroluminescent element (OLED) is actively developed. Here, assume that an OLED includes both of an element utilizing light emission (fluorescence) from singlet exciton and an element utilizing light emission (phosphorescence) from triplet exciton. In this specification, the OELD is indicated as an example of a light emitting element. However, another light emitting element may be used.
The OLED is composed of a pair of electrodes (cathode and anode) and an OLED layer sandwiched therebetween and a laminate structure is generally used. Typically, there is a laminate structure (hole transporting layer, light emitting layer, and electron transporting layer) proposed by Tang, Eastman Kodak Company.
In addition to such a structure, there is a structure in which (a hole injection layer, a hole transporting layer, a light emitting layer, and an electron transport layer) or (a hole injection layer, a hole transporting layer, a light emitting layer, an electron transport layer, and an electron injection layer) are laminated in the stated order. In the present invention, any of those structures may be employed. In addition, the light emitting layer may be doped with a fluorescent pigment.
In this specification, all layers provided between the anode and the cathode are generically called an OLED layer. Thus, the hole injection layer, the hole transporting layer, the light emitting layer, the electron transport layer, and the electron injection layer all are included in the OLED layer. A light emitting element composed of the anode, the OLED layer, and the cathode is called an OLED.
FIG. 2 shows a structural example of a pixel portion of an active matrix OLED display device. Gate signal lines (G1 to Gy) to which a selection signal is inputted from a gate signal line driver circuit each are connected with the gate electrode of a switching TFT 201 in each pixel. Also, with respect to the source region and the drain region of the switching TFT 201 in each pixel, one is connected with one of source signal lines (S1 to Sx) to which a signal is inputted from a source signal line driver circuit, and the other is connected with the gate electrode of an OLED driving TFT 202 and one electrode of a capacitor 203 in each pixel. The other electrode of the capacitor 203 is connected with one of power supply lines (V1 to Vx). With respect to the source region and the drain region of the OLED driving TFT 202 in each pixel, one is connected with one of the power supply lines (V1 to Vx), and the other is connected with one electrode of an OLED 204 in each pixel.
The OLED 204 has an anode, a cathode, and an OLED layer provided between the anode and the cathode. When the anode of the OLED 204 is connected with the source region or the drain region of the OLED driving TFT 202, the anode of the OLED 204 becomes a pixel electrode and the cathode thereof becomes a counter electrode. Conversely, when the cathode of the OLED 204 is connected with the source region or the drain region of the OLED driving TFT 202, the cathode of the OLED 204 becomes a pixel electrode and the anode thereof becomes a counter electrode.
Note that a potential on the counter electrode is called a counter potential in this specification. A power source for providing the counter potential to the counter electrode is called a counter power source. A potential difference between a potential on the pixel electrode and a potential on the counter electrode is an OLED drive voltage. The OLED drive voltage is applied to the OLED layer.
Note that in this specification, the switching TFT is an N-channel TFT and the driving TFT is a P-channel TFT. In addition, with respect to the electrodes of the OLED, one connected with the driving TFT is assumed as an anode and the other is assumed as a cathode. However, this does not mean that a combination other than the above cannot be realized. Therefore, other combinations are also possible.
With respect to a gradation display method for the above OLED display device, there are a constant current analog gradation method and a constant voltage time gradation method. In addition to them, there is a constant current time gradation method. Here, the above two types will be described. With respect to the definition of words, “constant current drive” means that the device is driven at a constant current during a period for which a video is held, such as one frame period and does not mean that the device is always driven at the same current. The same is applicable to the term “constant voltage drive”. FIG. 10A is a conceptual diagram showing the constant current drive and FIG. 10B is a conceptual diagram showing the constant voltage drive. According to the constant current drive, the OLED driving TFT is used as a voltage control type current source and a gate voltage of the driving TFT is controlled to flow a necessary current into the OLED. The constant voltage drive is a drive method in which the OLED driving TFT is used as a switch and the power supply line and the OLED are short-circuited when necessary to emit light from the OLED.
First, the constant current analog gradation method for the OLED display device will be described. FIG. 3 is a block diagram of a constant current analog gradation type display device. In addition, FIG. 4 is its timing chart. Hereinafter, a description will be made using FIG. 3. First, when a gate start pulse GSP and a gate clock pulse GCL are inputted to a shift register 304, a shift pulse is formed in the shift register 304. The shift pulse is outputted to a gate signal line through a buffer circuit 305. The gate signal lines are selected in succession according to the shift pulse. While the gate signal line is selected, a source start pulse SSP and a source clock pulse SCL are inputted to a shift register 302 of a source signal line driver circuit. Thus, a shift pulse is formed in the source shift register 302 and outputted to control terminals of analog switches 312 and 313 though a buffer circuit 303. When the analog switches 312 and 313 are selected in succession, an analog video signal line 314 and source signal lines 306 and 307 are short-circuited in succession so that analog video signals are sampled in succession for the source signal lines. The sampled analog video signals each are inputted to the gate of the OLED driving TFT through one of the source signal lines 306 and 307 and the switching TFT in each pixel.
As described above, the amount of light emission of the OLED is controlled according to the analog video signal, and gradation display is conducted by controlling the amount of light emission. Thus, according to the constant current analog gradation method, the gradation display is conducted according to a change in potential of the analog video signal inputted to the source signal line.
In the constant current analog drive in which a drain current corresponding to Vgs flows into a driving TFT, a TFT is generally operated in a saturation region. FIG. 5A shows the operation of the TFT. The saturation region is a region indicating Vds>Vgs, a region in which a change in drain current is small as compared with a change in Vds. This region is used as a pseudo constant current.
Next, the constant voltage time gradation method will be described. According to the time gradation method, a digital signal is inputted to a pixel to select a light emitting state or a non-light emitting state of the OLED, and the gradation is represented according to the accumulating total of OLED light emitting periods per frame period. Note that the principle of time gradation is described in JP 2001-159878 A.
FIG. 7 is a block diagram of a display device 701 using the constant voltage time gradation method. In addition, FIG. 8 is its timing chart. Hereinafter, a description will be made with reference to FIG. 7. The gate signal line driver circuit is the same as in the case of the analog gradation drive and therefore the description is omitted here. A source signal line driver circuit is composed of a shift register circuit 702, a buffer circuit 703, a first latch circuit 704, and a second latch circuit 705. A source start pulse SSP and a source clock pulse SCL are inputted to the shift register circuit 702. The shift register circuit forms a shift pulse in response to those pulses. The shift pulse is inputted to the first latch circuit 704 through the buffer circuit 703. When the shift pulse is inputted to the first latch circuit, the first latch circuit latches a digital gradation signal. When a shift of one line is completed, digital video data corresponding to one line is stored in the first latch circuit 704. During a retrace period after that, a latch pulse is inputted to the second latch circuit 705. In response to the latch pulse, the digital video data stored in the first latch circuit 704 is transferred to the second latch circuit 705 and outputted to source signal lines 708 and 709. Then, video data corresponding to a next line is stored in the first latch circuit 704. Such operation is repeated so that digital video data is outputted to the source signal lines 708 and 709 in succession.
With respect to the conventional OLED display device as described above, there are the following problems.
First, in the constant current analog drive type display device, as described above, voltage-current conversion is conducted by the OLED driving TFT. Thus, when mobility and a threshold value of the TFT are varied, these variations cause a variation in drain current. Therefore, when an in-plane variation of the TFT is large, it appears as display nonuniformity. For example, if the mobility of the TFT is varied by 10%, luminous intensity is also varied by 10%. In addition, the threshold value is varied by 0.1 V, this also results in a luminous intensity variation of about 10%. As for the threshold and the mobility, these have independent variations, thereby causing a variation of about 14% in total. Accordingly, establishment of a method for alleviating variations in TFT characteristics is desired. The problem described above is described in JP 2000-221903 A and the like.
On the other hand, in the constant voltage time gradation drive, the influence of a variation in TFTs on display is small. When the TFT is operated in a linear region, the term of the threshold value is a first power term and Vgs is set large. Thus, even if there is a variation of 0.1 V in threshold value, a luminous intensity variation of only about 1% is caused. In addition, even if a variation in mobility is 10%, negative feedback is generated between Vgs and a forward direction voltage of the OLED. Therefore, a variation in current is suppressed to be reduced to 5% or less.
However, in the constant voltage time gradation drive, there is a problem such as deterioration of the OLED with time. A change in OLED with time will be described with reference to FIGS. 12A and 12B. When the OLED is driven, two deterioration phenomena appear. A first deterioration phenomenon is a reduction in intensity. FIG. 12A shows its example. A light emission intensity of the OLED is reduced with time. A period of time until when the intensity is reduced by half is assumed as a life time. The life time depends on the intensity but is at present generally 1000 hours to several 1000 hours at about 200 cd/m2. As shown in FIG. 12B, when the deterioration is caused, a slope of a current-intensity characteristic is reduced.
Also, a second deterioration phenomenon is an increase in forward direction voltage. As shown in FIG. 13A, when the same current continuously flows, the forward direction voltage is being increased. FIG. 13B shows a voltage-current characteristic. As shown in FIG. 13B, the characteristic is shifted from the left to the right before and after the deterioration. FIGS. 9A to 9C show changes in operating point of the constant current drive and that of the constant voltage drive. According to the constant current drive, only in the former case of a reduction in light emission efficiency, the deterioration appears on display. As shown in FIG. 9A, when there is a sufficient margin for Vds of a TFT, an increase in forward direction voltage of the OLED is absorbed thereby so that it does not appear on display. On the other hand, as shown in FIG. 9B, according to the constant voltage drive, an increase in forward direction voltage causes an increase in value of current change ΔI. In the case of the constant voltage drive, an effect of both a decrease in current and a reduction in light emission efficiency is caused. Thus, there is a problem that the deterioration appears to increase.
In a display device, a light emission time of a pixel is changed according to a location. With respect to a location such as a position of an icon, a cumulative light emission time is long so that rapid deterioration is caused. When the entire surface of a screen is displayed at uniform luminous intensity, the luminous intensity is reduced in a location where the deterioration is rapid. Thus, there is a problem that only such a portion is sensed as burn-in.